Various types of modulated data transmission are well known. Examples of modulation techniques for the transmission of analog information include amplitude modulation and frequency modulation. Special modulation techniques are also well known for the transmission of digital data, including frequency shift keyed (FSK) modulation and phase shift keyed (PSK) modulation. As is well known in the art, in FSK modulation a number of cycles at a first frequency represents a "0" digital value, and a number of cycles at a second frequency represents a "1" digital value. PSK modulation uses the same carrier frequency for both a "0" value and "1" value, with different phase angles corresponding to the different digital values. A conventional phase difference between "0" and "1" states in PSK modulation is 180.degree. .
In the petroleum industry, particularly in downhole operations such as drilling into the earth in the exploration and production of petroleum products, the communication of data concerning the operation is becoming increasingly important. Since modern drilling is necessarily done to greater depths (and at greater cost) in order to exploit less readily available reservoirs, the cost of drilling operations is increasing at a rapid rate. In order to ensure the highest degree of success in such operations, and to avoid potential problems during drilling, real-time knowledge of downhole conditions such as drilling parameters, bit location, bit direction, and information concerning the surrounding geology, is highly desirable.
My copending U.S. applications Ser. No. 554,022 now abandoned in favor of continuation application Ser. No. 746,309, filed Aug. 14, 1991, 554,030, both filed Jul. 16, 1990, assigned to Atlantic Richfield Company and incorporated herein by this reference, describe examples of transducers for communicating data from downhole to the surface by way of acoustic vibrations in the drill string, such communication commonly referred to as stress wave telemetry. As discussed therein, and as is true in other data communication environments, the transmission medium has a non-ideal frequency response to the signals being transmitted therethrough. In the case of data transmission by acoustic vibrations in the drill string (such transmission referred to as stress wave telemetry), significant stopbands exist at harmonic frequencies defined by the velocity of the particular vibrations in the drill string and the lengths of the drill string sections. As a result, FSK modulation of acoustic vibrations may have problems where one or both of the transmission frequencies are at or near a stopband for the drill string. Since the drill string characteristics can change as a result of the drilling operation (i.e., longer drill string sections, heavier pipe, heavier mud, etc.), assumptions that both FSK frequencies are in passbands for the drill string may not remain valid throughout the duration of the operation.
Another stress wave telemetry system is disclosed in copending U.S. Pat. application Ser. No. 183,231 filed Apr. 21, 1988, now U.S. Pat. No. 4,825,159, also assigned to Atlantic Richfield Company, and incorporated herein by this reference. This system uses equipment outside the periphery of the drill string, such as solenoids and eccentric motors, to similarly vibrate the drill string in a manner corresponding to the desired data.
PSK modulation would thus appear to be attractive in situations such as stress wave telemetry where non-ideal frequency response is presented by the communication medium. However, in the case of data transmission by acoustic vibrations through a drill string, the drill string tends to resist the phase changes necessary for communicating the information. A change in phase of the transmitted vibrations, indicating a change from one digital state to the other, will thus not appear instantaneously in the drill string. A series of transitional cycles are thus required in order for a change in phase (i.e., a change in data state transmitted) to take effect in this situation.
Conventional PSK demodulators are not capable of readily and accurately identifying phase changes which are not abrupt, for example which include a number of transitional cycles in making the phase change. It has been observed, however, from viewing oscilloscope and other representations of actual vibrations transmitted along a drill string, that PSK signals including such transitional cycles still contain the transmitted data.
In addition, noise and other spurious events during stress wave telemetry or other transmission can cause the loss of partial or full cycles of the PSK modulated data stream. When applied to conventional demodulators, such lost cycles and portions of cycles create significant error, due to loss of synchronization between the modulator reference signal and the input data signal.
It is therefore an object of this invention to provide a system for demodulating PSK modulated data which includes transitional cycles at the time of data state transitions.
It is a further object of this invention to provide such a system which provides its own reference signal in a manner which compensates for transitional cycles and loss of cycles in the transmission.
It is a further object of this invention to provide an improved demodulator for PSK modulated information.
It is a further object of this invention to provide such an improved system and demodulator applicable to stress wave telemetry and other acoustic vibration communication systems.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to this specification together with its drawings.